Investment casting, which has also been called lost wax, lost pattern and precision casting, is used to produce high quality metal articles that meet relatively close dimensional tolerances. Typically, an investment casting is made by first constructing a thin-walled ceramic mold, known as an investment casting shell, into which a molten metal can be introduced. Shells are usually constructed by first making a facsimile or pattern from a meltable substrate of the metal object to be made by investment casting. Suitable meltable substrates may include, for example, wax, polystyrene, or plastic.
Next, a ceramic shell is formed around the pattern. This may be accomplished by dipping the pattern into a slurry containing a mixture of liquid refractory binders such as colloidal silica or ethyl silicate, plus a refractory powder such as quartz, fused silica, zircon, alumina, or aluminosilicate, and then sieving dry refractory grains onto the freshly dipped pattern.
The steps of dipping the pattern into a refractory slurry to form a layer, and then sieving onto the freshly dipped pattern dry, refractory grains as an added “stucco” layer, may be repeated until the desired thickness of the shell is obtained. However, it is preferable if each coat of slurry and refractory grains is air-dried before subsequent coats are applied.
The shells are built up to a thickness in the range of about ⅛ to about ½ of an inch (from about 0.31 to about 1.27 cm). After the final dipping and sieving, the shell is thoroughly air-dried. The shells made by this procedure have been called “stuccoed” shells because of the texture of the shell's surface.
The shell is then heated to at least the melting point of the meltable substrate. In this step, the pattern is melted away leaving only the shell and any residual meltable substrate. The shell is then heated to a temperature high enough to vaporize any residual meltable substrate from the shell. Usually before the shell has cooled from this high temperature heating, the shell is filled with molten metal. Various methods have been used to introduce molten metal into shells including gravity, pressure, vacuum and centrifugal methods. When the molten metal in the casting mold has solidified and cooled sufficiently, the casting may be removed from the shell.
The complete removal of the cast or shell surrounding the part, which is sometimes called the “knock-out”, often requires long, dirty, dangerous, and costly processes, using sandblasting, mechanical vibration, or caustic baths.
Investment casting molds must withstand significant mechanical and drying stresses during their manufacture. Ceramic shells are designed having high green (air dried) strength to prevent damage during the shell building process. Once the desired mold thickness is achieved, it is dewaxed and preheated to approximately 1800° F. At this point, it is removed from the high temperature furnace and immediately filled with liquid (molten) metal. If the mold deforms while the metal is solidifying (or in a plastic state), the casting dimensions will likely be out of specification. To prevent high temperature deformation, molds are designed to have substantial hot strength. Once the casting is solidified and cooled, low fired strength is desired to facilitate the knock out or removal of the ceramic mold from the metal casting.
Most investment casting molds contain significant quantities of silica. The silica usually starts as an amorphous (vitreous) material. Fused silicas and aluminosilicates are the most common mold materials. When exposed to temperatures above approximately 1800° F., amorphous silica devitrifies (crystallizes) forming beta cristobalite. Cristobalite has low (alpha) and high (beta) temperature forms. The beta form has a specific gravity very close to that of amorphous silica so mold dimensions remain constant and stresses associated with the phase transformation are minimal. Upon cooling, beta cristobalite transforms to the alpha form. This phase transformation is accompanied by an approximate 4% volume change that creates numerous cracks in the shell, thereby facilitating mold removal. Cristobalite phase transition reduces the fired strength of silica containing investment casting molds.
Although investment casting has been known and used for thousands of years, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high-quality, precision castings, there continuously remains a need to develop new ways to make investment casting shells more efficiently, more cost-effective and defect-free. For instance, if shell strength was maintained to the point of metal solidification, followed by a reduction in strength as the shell cools, improvements in productivity could be realized through improved knock out (shell removal). This is particularly desirable for non-ferrous alloys, e.g. alloys of aluminum, copper and magnesium, because their melting and pouring temperatures are insufficient to promote cristobalite formation and easy knock out.
The knock out is especially difficult when the part presents a blind hole or a small cavity in which the ceramic is under compression. The compression occurs during the cooling of the metal parts, which in general have a higher coefficient of thermal expansion (CTE) than the ceramic shells. This effect is especially accentuated in non-ferrous castings because of the high coefficient of thermal expansion of this metal (>18×10−6 m/m).
Non-ferrous castings produced by investment casters are rather fragile, so they are cleaned by water or sand blasting, compared with the aggressive shot blast and vibratory cleaning for steel and high temperature alloy castings. Residual ceramic on steel castings is dissolved away using concentrated acids and bases or molten salt baths. Chemical incompatibility excludes their use on aluminum and magnesium castings. If a binder was developed having low fired strength and associated easy knock out properties upon exposure to temperatures at or below 1800° F., aluminum casting cleanup could be greatly improved.
Accordingly, it would be desirable to provide an improved method of removing an investment casting shell surrounding a metallic part.
Batllo U.S. patent application Ser. No. 10/337,799 addresses this issue by adding a salt of alkali or alkaline earth metal to at least one of the layers of an investment casting shell. The addition of a salt of alkali or alkaline earth metal effectively improves the removal of the investment casting shell surrounding a metallic part by reducing the shell strength, particularly after firing.
Such addition of alkaline earth metal salt, such as calcium carbonate added to the “green” investment casting shell, can provide a weakening effect that results in a two to threefold increase in erosion speed of the fired shell that surrounds the solidified metal casting, as measured by blasting tests such as sandblasting.